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Those are good fish choices. I recommed buying captive bred clown fishes because they are much hardier, will accept most foods, are usually disease/parasite free, are use to aquarium life, are more friendly to people(sometimes will eat out of your hand), and you will know the age of the fish, unlike wild caught clowns.
go with the clowns (2) first. They are very hearty and very friendly. the tang needs to be last, I would say the fire fish should be just after the clowns, and then your royal gramma
Clowns, then give the skittish firefish a good long time to get acclimated before adding anything else that may mess with them. Royal Gramma then the Tang. Excellent choices and should set you up for success.